The present invention concerns a novel bath and method for electroplating high-purity gold and various gold alloys. Typically, such deposits are produced from baths that contain the cyanide and/or the phosphate radical, although other electrolytes, such as those which are based upon the citrate radical, have certainly been used to good advantage. It is of course common to add brighteners, hardeners, chelating agents, and other ingredients to such solutions to provide a variety of modifications in the nature of the deposit and improvements in the operating characteristics of the bath.
The prior art shows the inclusion of carbonate compounds and tartrate compounds in gold plating baths for a variety of purposes. In Spreter et al U.S. Pat. No. 2,724,687, for example, the use of potassium carbonate is disclosed for pH adjustment in a gold alloy bath. An acid gold bath containing tartaric acid and a base metal tartrate salt is taught in Rinker et al U.S. Pat. No. 2,905,601, and acid gold baths containing tartaric acid are disclosed both in Ostrow et al U.S. Pat. No. 2,967,135 and also in Rinker et al U.S. Pat. No. 3,104,212.
A tartaric acid-containing gold plating bath, which may be operated at a pH of 6.1 to 10.5, is disclosed in Camp U.S. Pat. No. 3,397,127, and Shoushanian U.S. Pat. No. 3,475,292 teaches the use of potassium carbonate or potassium tartrate as a buffering and conducting salt in a gold plating bath operated at a pH of 8 to 12 or higher. Haynes et al U.S. Pat. No. 3,475,293 discloses the use of potassium carbonate in a variety of metal baths, Freedman et al. U.S. Pat. No. 3,598,706 teaches the inclusion of tartrates in acid gold plating baths, and Smith U.S. Pat. No. 3,666,640 utilizes tartaric acid as a chelating agent in a gold sulfite formulation.
In U.S. Pat. No. 3,893,896, Korbelak et al teach a gold plating solution containing, among other ingredients, an alkali gold cyanide, a weak aliphatic acid, a non-depositing metallic compound, and a metallic hardener. The weak acid may be tartaric, the non-depositing metal may be introduced as a carbonate compound, and the metal hardener may be added as the tartrate; the pH range of the bath is disclosed to be about 3.7 to 4.8. Lerner U.S. Pat. No. 3,926,748 teaches a neutral gold and antimony bath, which includes potassium tartrate and antimony potassium tartrate. The incorporation of alkali metal tartrates in a gold alloy bath that operates at a pH between about 8 and 10 is disclosed in Moriarty et al U.S. Pat. No. 4,121,982.
The prior art also teaches the use of the tartrate and carbonate compounds in silver plating baths. In particular, such formulations are disclosed in Ruemmler U.S. Pat. No. 2,555,375. Similarly, the combination of tartar emetic and potassium carbonate for plating silver from a cyanide bath is taught by Gardner in U.S. Pat. No. 3,219,558, and he discloses in U.S. Pat. No. 3,362,895 using antimony potassium tartrate in a weak acid/salt combination, as a buffering system in a neutral silver bath.
Despite the foregoing, a demand exists for a gold plating bath that is capable of producing electrodeposits of high purity, and particularly one that is capable of producing deposits of sufficient purity to meet the requirements of the semi-conductor industry, using either continuous or pulsed direct current. A need also exists for such a bath of relatively low specific gravity, which is capable of operating under high current density plating conditions at relatively low gold concentrations, and which has a high tolerance to contamination, such as from copper. Finally, there is a demand for a bath having the foregoing features asd advantages, from which gold can be alloyed with various metals to produce a variety of characteristics in the deposit, and to extend the range of brightness exhibited thereby.
Accordingly, it is a primary object of the present invention to provide a novel plating bath which is capable of producing highly pure gold electrodeposits, and a novel method utilizing the same.
It is also an object of the invention to provide such a novel bath and method in which the bath is capable of operating under high current density plating conditions, and is of relatively low gold concentration and specific gravity.
It is a more specific object of the invention to provide such a bath and method which are capable of producing electrodeposits of sufficient purity to meet the requirements of the semi-conductor industry.
It is a further object of the invention to provide such a bath which has a high tolerance to contamination, and which can produce gold alloy deposits of varying karat values and of extended brightness range.